1. Field of the Invention
This invention relates to a transducer for use in an accelerometer or other force transducer and in particular to a micro-accelerometer fabricated as a chip from a semiconductor material wafer using manufacturing techniques analogous to those of integrated circuits. Such micro-accelerometers find applications in the motor and aerospace industries as inputs for control systems; in the motor industry the uses include triggering air bags and as inputs to suspension systems.
2. Description of the Related Art
Micro-accelerometers can be single axis devices, sensitive in a single direction, which are then assembled to form a three dimensional unit. Three axes micro-accelerometers, constructed from a single device which is sensitive in all three dimensions, have structures which require thick slices of base material.
Accelerometers may have to survive over-shocks of hundreds of times their normal operating range and usually employ deflection stops incorporated into the structure to prevent damage. Accelerometers built from single-crystal semiconductor material exhibit very low mechanical loss and need to have controlled damping in order to ensure high-fidelity transduction of acceleration. In "Accelerometer systems with Self-Testable Features" by Allen, Terry and De Bruin, Sensors and Actuators, 20 (1989) 153.about.161, incorporated herein by reference, a single axis accelerometer having a double cantilever structure is disclosed. Such a device has a seismic mass supported through four silicon springs and the mass can move in a pure translational mode as shown in FIG. 1a. Unwanted rotational modes are also possible as shown in FIGS. 1b and 1c. These rotational modes may be caused by forces which are not perpendicular to the surface of the mass, or by a lack of symmetry in the mass. There will be a tendency for at least one corner of the mass to deflect upwards and the mass will tend to twist, placing an uneven strain on the silicon supports.
EP 322093 A discloses a deflectable seismic mass constructed from a single wafer of silicon. The mass moves rectilinearly in response to a force applied perpendicularly to the surface of the wafer. All of the embodiments disclosed show masses in which movement in the {1,1,1,} plane is constrained and which deflect in a plane perpendicular to the {1,1,1} plane and the surface of the wafer (the {1,0,0} silicon plane). The mass, when subjected to a force which is not perpendicular the {1,0,0,} silicon plane, tends to move and rock through the deflectable plane. The mass of FIG. 5 of EP 322093 is constrained to rotate about a line parallel to the {1,1,1} silicon plane, positioned at the intersection of the planes defined by flexible members 24" and 26". Thus the motion of the mass is coupled in two of the three dimensions and the device is thus unsuitable for applications where a single axis transducer is required. The devices disclosed are also unsuitable for use in a flat, single wafer device which is sensitive in all three dimensions. As before, a lack of symmetry in the mass will cause unwanted rotational movements.
An example of a three axes single crystal force transducer is shown in WO 94/12886.